1. Field of the Invention
This invention relates generally to adaptive forward error correction coding in a data communication system and, more particularly, to a method of organizing data cells into a fixed size frame for transmission in the downlink from a processing satellite to a plurality of earth terminals.
2. Discussion of the Related Art
A downlink transmission from a spot beam processing satellite is typically a single access time division multiplexed (TDM) stream that is organized into discrete units known as frames. Generally, these frames encapsulate information which has been processed (at the satellite) to add redundancy for error correction at the earth terminal. In processing satellites, the forward error correction coding scheme typically applies an inner code and an outer code separated by an interleaver. To provide additional margin for weather effects and/or compensate for variations in gain across an antenna footprint, the rate used for error control coding may be varied from frame to frame.
To avoid complex processing in the transmission and reception of these frames, it is desirable that the size of the frame be invariant to changes in coding rates, otherwise more complex circuitry may be required to handle differently coded frames. In addition, variable size downlink frames present complications for the earth terminal electronics in deriving uplink synchronization from the downlink transmissions.
In accordance with the present invention, a method is provided for organizing a plurality of data cells into a fixed size frame for transmission in the downlink of a processing satellite, comprising the steps of: (a) determining an inner coding rate for a first set of data cells; (b) forming a group of codewords by applying an outer code to the first set of data cells, such that the number of codewords is proportional to the inner coding rate; (c) entering the group of codewords row wise into an interleaving array; and (d) applying an inner code column wise to the group of codewords, thereby forming a fixed size frame body.